I am not one to go and test all my lenses by taking "test shots" of targets and seeing the lines per inch or any other meaning less statistic. on the other hand i do a more practical test: i use the lens the way i would on a daily basis.

The first thing that i noticed on this lens is the amazing build quality; the finish is nice and smooth yet not plasticky and when you hold it in your hand you know that you are holding something serious. When i first started using it the zoom ring was a bit tight but has loosened to a comfortable level since then. the focus ring had no such problems.

The focusing is smooth in manual and fairly fast in auto; although it really doesnt have to be very fast because of its wide nature. one point of interest is that the length of the lens does not change when focusing or zooming, although the front element moves about 3mm back and forth when zooming.

The lens felt very comfortable on my 20d. the lens hood is a pettle type (its felt lined to boot!) and does a pretty good job except for when the sun, or a bright light source, is right on the edge of the frame.

The Pictures! the pictures taken with this lens are quite amazing. the constant f/4 aperature is extremely usable and i did not notice very much vignetting (even at f/4). The color and saturation are quite impressive although i ended up messing with the colors and saturation in PP to bring the most out of the lens. I did not notice anything good or bad about the contrast -- it just works, nothing to get excited about for the contrast.

The build of this lens is very good. it has a very long travel focusing mechanism that is smooth and precise throughout the entire range. The front of the lens extends about the entire length of the lens, making the lens double the length when focused to its closest distance.

The image quality is top notch. Wide open it has great sharpness from corner to corner. stopped down it gets even better. i spent a week in Yellowstone / Grand Tetons and this was the lens i found myself using the most.

I have used this lens with extension tubes and the mt-24EX twin light. With tubes you have to get really close to your subject but i had no problem, even when photographing bugs.

Samples:

1. Blossom -- this one was done with 68mm of extension and was very hard to focus, however the look worked. this is not a good example of wide open sharpness

this is by far my favourite canon lens. i use it almost exclusivly for taking panoramas. the build is on par with a tank. very sturdy and pretty substantial in your hand.

the FOV on a 1.6x crop camera is perfect for everyday shooting. this lens almost never leaves my camera. the only down side is that it produces a 'hotspot' when shooting infrared. this limits its uses for infrared shooting. not bad considering the speed of the lens.

subject isolation and bokeh is sooo sooo sooo good.
here are some samples of the lens:

this is the first IS lens i have used. the greatest thing about this lens is that it covers such a wide range of focal lengths in such a small package. throw in IS and you got a near perfect walkaround lens. since i have aquired the 35L i havent been using this lens quite as much. i feel that the image quality this lens gives just isnt good enough for the type of shooting i do. the speed of this lens is another neg for me. i guess f/1.4 spoiled me. =)

overall a nice entry lens for someone that doesnt want to bother with changing lenses or that is going to be taking snapshots of which the Image quality is not of much concerne. but! dont get me wrong; under the right circumstances this lens can produce amazing images...

I purchased this lens to replace my Tokina 12-24 f/4 when i moved from the 20D to a 5D2. it provides a slightly longer and wider fov than the tokina on the 20D. the build quility is amazing. no creeks no jiggles, just a solid lens.

both the focus ring and the zoom ring are very smooth. ive had no issues with them and are much nicer than the tokina 12-24 than it is replacing.

now to the image quality. to be upfront, the image quality is really good. combined with the 5D2 ive gotten some amazing images. the flare you get on this lens is actually quite nice. long star shaped lines coming from the light source. it has mostly been used at 17mm and i have yet to see anything that makes me regret the purchase. yes, there is distortion... its a wide angle zoom.... but if you are shooting RAW (which you should be) then a quick check of the Lens Profile Correction box in Camera RAW will take care of that.

Great lens.

the only issues that ive had are with usability related to the lens hood and circular polarizers. the lens hood is very big and it is difficult to operate a polarizer with it on. there are ways around this. pentax came up with a great solution for some of its lenses.

see this image. (they add a cut out to the lens hood) http://i01.i.aliimg.com/img/pb/776/581/285/285581776_785.jpg